﻿<p>This entity describes surface members with varying section properties.  The properties are provided by means of
<em>Pset_StructuralSurfaceMemberVaryingThickness</em>
via <em>IfcRelDefinesByProperties</em>, or by means of aggregation:  An instance of <em>IfcStructuralSurfaceMemberVarying</em> may be composed of two or more instances of <em>IfcStructuralSurfaceMember</em> with differing section properties.  These subordinate members relate to the instance of <em>IfcStructuralSurfaceMemberVarying</em> by <em>IfcRelAggregates</em>.</p>

<blockquote class="note">
NOTE&nbsp; It is recommended that structural activities (actions or reactions) are not connected with aggregated <em>IfcStructuralSurfaceMemberVarying</em> but only with the <em>IfcStructuralSurfaceMember</em>s in the aggregation.  That way, difficulties in interpretation of local coordinates are avoided.
</blockquote>

<blockquote class="history">
HISTORY&nbsp; New entity in IFC2x2.
</blockquote>

<blockquote class="change-ifc2x4">
IFC4 CHANGE&nbsp; Use definition changed and attributes deleted.
</blockquote>

<p><b><u>Coordinate Systems</u></b>:</p>

<p>See definitions at <em>IfcStructuralItem</em> and <em>IfcStructuralSurfaceMember</em>.  The local coordinates of an aggregate are generally undefined since continuity of local coordinates of the parts is not ensured.</p>

<p><b><u>Material Use Definition</u></b></p>

<p>In case of aggregation, only the individual parts (direct instances of <em>IfcStructuralSurfaceMember</em>) carry material and thickness information.  Otherwise, definitions at <em>IfcStructuralSurfaceMember</em> apply.</p>